Testing in recent years has confirmed that the double zincate process is still the most reliable for the largest variety of alloys. There have been improvements in the chemistry of zincate solutions. Also, acid-zinc-immersion processes have been introduced that have some significant advantages over alkaline zincate processes. And, there is now a better understanding of the factors influencing adhesion.
Good adhesion is dependent on having a clean surface free from soils, oxides, particulate matter and embedded materials. Non-etch or mild etch alkaline cleaners work well for many soils. However, more aggressive cleaners may be needed for heavy soils and scales. Etching can result in poor corrosion resistance due to the formation of capillary voids around alloying constituents. These voids can draw in cleaning solution that bleeds out in the hot electroless nickel (EN) plating solution. The aluminum metal expands at the higher temperature, allowing the contaminants to prevent plating. The result is a pit. Alloying constituents creates electrochemical corrosion cells on the surface of the aluminum. Etching causes accelerated corrosive action at these cell sites. Grain boundaries give rise to electrochemical corrosion cells, resulting in preferential etching along these boundaries.
Why use zinc immersion?
Isn't it true that immersion (chemical replacement) deposits have generally poor adhesion? What are the advantages and disadvantages of using a zinc or tin immersion process? Addressing these questions may help you understand how and why these processes work and how plating deposit adhesion is affected by the processing steps.
Aluminum and its alloys have a high affinity for oxygen. This means that clean, bare aluminum forms an oxide film rapidly. It is difficult to plate onto an oxide film with good adhesion. The natural oxide film formed on aluminum helps protect it in normal outdoor environments. However, to electroless nickel plate (or any other plate) onto aluminum alloys successfully with good adhesion, the oxide film must be removed and remain so during plating deposition. EN plating is selected because of its ability to protect aluminum from hostile environments and to make a hard, wear-resistant surface that allows aluminum to be used for applications not otherwise suitable. EN deposits have a pleasing appearance and can be used for decorative as well as functional applications.
Zinc and tin immersion processes produce coatings on clean, deoxidized aluminum and protect the aluminum from oxidation. The zinc and/or tin immersion solutions dissolve the thin oxide left from the deoxidizing and rinsing steps. Then they attack the aluminum by dissolving it primarily at the grain boundaries and around alloying constituents. Note that these areas are the electrochemical corrosion sites where there is a potential voltage difference between two adjacent areas. Excessive attack on the aluminum will cause pits when EN plated. Care must be taken not to over etch in any of the preparatory processes.
|TABLE I—Double Zincate Process|
Immersion plating, including zinc immersion deposits such as from zincate solutions and acid-zinc-immersion deposits do not have good adhesion. There may be exceptions. Experiments with EN plating aluminum alloys without a prior zinc-immersion deposit show slow initiation of the deposition process and poor adhesion.
Zinc coatings dissolve
The zinc deposit formed by the immersion processes is dissolved in the EN plating solution under normal operating conditions and temperatures. The dissolving or not dissolving of zinc deposits has been the subject of controversy for some years. The question of dissolving or not lies in whether or not it is possible to find solution formulations and operating conditions under which maximum dissolving of the zinc layer occurs and whether this dissolving is useful. It seems to be agreed that the thinnest possible zinc-coating layer is desirable.
Vangelova et al.12 studied deposits from acid EN solutions and, using Auger electron spectroscopy, found no trace of zinc at the interface of aluminum to nickel. Deposition started on the pure oxide-free aluminum-based alloy surface. Armyanov1 stated that the dissolution of zincate coatings could be controlled by varying several plating parameters, including pH, temperature, ligand species and concentration. Because the zinc film dissolves in acidic hypophosphite electroless solutions, an EN strike bath was introduced into the preplate process cycle. Another evidence of dissolution of zinc, although it does not prove complete dissolution, is that monitoring the zinc content of EN solutions that are used exclusively for plating onto aluminum using the double zincate process indicates a linear increase of zinc with use of the plating solution.
It is theorized that when the EN plating solution dissolves the zinc, electrons are contributed to the reaction, lowering the effective deposition potential for nickel. These electrons add to the potential from sodium hypophosphate to initiate nickel deposition over the surface of the aluminum. It is believed that nickel deposits by immersion initially followed catalytic deposition. The intimate contact and quick initiation result in nickel plating directly onto the aluminum without interfering oxide or zinc layers. Thus, adhesion is enhanced. When a plated deposit can start by growing in the crystal sites of the basis metal, high adhesion values can be achieved.
Other factors influencing adhesion
Stress within the EN deposit, sometimes called intrinsic or internal stress, produces a force at the interface boundary between the aluminum and nickel deposit. If stress is high enough and the adhesion is not perfect, the deposit can separate or blister. Stress in the deposit can be tensile or compressive. Typical values range from about 8,000 to 25,000 psi tensile for some mid-phosphorus deposits. High-phosphorus alloys tend to be compressively stressed from a few hundred to about 4,000 psi. Low-phosphorus deposits tend to be slightly compressively stressed. Both low-and high-phosphorus deposits tend to be more ductile than mid-phosphorus deposits; thus, they may withstand thermal forces somewhat better.
When plated aluminum is subjected to elevated temperatures or cold conditions, forces are generated by the materials' difference in the coefficient of expansion. These forces added to intrinsic stress can cause separation of the deposit.
There are four items in the final internal stress value in EN coatings deposited on aluminum: As plated, thermal, due to hydrogen desorption and because of stress relaxation. The desorption of hydrogen, occluded during the EN plating, makes internal stress more tensile at room temperature. This process should continue during the annealing also. In addition, during the annealing the absolute internal stress value decreases because of stress relaxation. The acting stress in the process of stress relaxation includes all three aforementioned variables. The thermal stress of cooling to room temperature is added to the relaxed stress. In industrial practice, a low-temperature bake (130 to 275F) for about two hours is used to relax stress and desorption of hydrogen, improving adhesion at the same time.
As EN plating solutions are used, the solution becomes more aggressive in attacking aluminum. The result is a lessening of the adhesion. Blisters or peeling of the deposit can occur. The reason for the increased aggressive nature of the solutions is speculated to be increases in anions such as sulfate. Nickel sulfate is added to most solutions to replenish nickel that has been deposited. The sulfate remains and continuously increases in the solution. Chlorides introduced by drag-in or use of tap water can also contribute to the attack on aluminum. Other factors may also contribute to poor adhesion.
|TABLE II—Preparation for Aluminum Casting Alloys
The result of these factors is shortened life of the solution when most or all of the production is aluminum. A common commercial practice is to plate aluminum parts first in a new solution and change to plating steel parts for the remainder of the solution life. Another practice is to use a special aluminum pre-plate to deposit a thin layer of nickel, then transfer to the plating solution. The accumulation of zinc in the main plating solution will be slowed considerably. Zinc contamination can slow the plating rate because it adds to the stabilizer effect. Most solutions are tolerant to about 60 to 100 ppm of zinc.
One disadvantage of the process is that zincate solutions attack and etch aluminum preferentially at grain boundaries and around alloying constituents. These represent corrosion cells. When aluminum is removed from the more insoluble-alloying constituents, there is a chance of leaving capillary paths. The greater the amount of etching, the greater the formation of capillaries. Capillaries are small openings that draw solution into them (zinc-ate solution). It is not possible to adequately rinse the zincate solution from the capillaries.
When the aluminum is submerged in the EN plating solution, the metal expands allowing zincate solution to bleed from these fissures. This prevents nickel plating in the small area, resulting in pits. Often, in addition to pits, nodules are formed in the adjacent area due to precipitation of salts from the high pH of the zincate solution entrapped in the capillary. Etching at the grain boundaries is less destructive since capillaries are not likely to form unless severe etching is encountered. However, small valleys can be etched, making the nickel deposit less smooth. In EN plating memory disks, any pits, no matter how small, may cause rejection since imperfections can cause loss of information. Non-etch cleaning, deoxidizing in non-etching acids and zincates that have minimal etching when short immersion times are used for this. The second zincate in the double zincate process forms a much thinner zinc coating since most of the etching has already taken place in the first zincate step. This thin coating assures good adhesion by making it easier for the plating solution to remove all the zinc plating initiates.
Aluminum memory disks
For memory disks and other applications where imperfections cannot be tolerated, acid-zinc-immersion processes provide good adhesion with less etching than from zincate solutions. The first zinc is removed, not in nitric acid, but mild oxidizing acid solution. Nitrates dragged in can contaminate the acid zinc immersion and cause adhesion loss. The second acid-zinc-immersion solution is used at a shorter time than the first, but both use longer times than for zincate solutions, but produce less etching. Thus, there is less opportunity for capillaries to form and less or no pitting or nodules.
Note that particulate matter in the bath can also cause nodules. High filtration rates through fine filters can remove nodule-forming particulates. Nodules and pitting can also result when the plating solution is out of balance or the bath is overwhelmed by too many particles. Stabilizers in the EN solutions serve to poison particles, preventing them from co-depositing. If not enough or the wrong types of stabilizers are used, or other chemical components of the solution are not adequate, pitting and nodules will result.
Stannate (tin) processes
Tin immersion solutions have been substituted for zincate solutions. Similar to zincate solutions, tin solutions etch in the same way. Stannate immersion solutions are particularly effective in certain alloys and not others. Zincates still encompass a larger variety of alloys.
In the case of aluminum casting alloys, zinc immersion processes are sometimes omitted successfully. Where zincates or acid-zinc-immersion solutions are used, a single zinc step is often adequate, since castings etch less than wrought alloys in these solutions.
Alkaline EN. Valova et al10, have developed an alkaline EN, pH 8, that deposits with a high phosphorus content, above 11%. The alkaline formulations are without free ammonium ions. Auger sputter-etch profiles have shown that at 88C operating solution temperature, the zinc coating was completely dissolved. However, at 93C and a higher deposition rate, zinc was present exactly at the interface. High-phosphorus-containing deposits have high magnetic conversion temperatures (more than 300C). Acidic acid citrate EN plating solutions containing alloying metals such as tin, copper or antimony have high phosphorus content.
Interface enrichment of these alloying metals can be avoided and adherent aluminum coating can be achieved. These ternary alloys have superior thermal stability with respect to magnetic properties. They remain non-magnetic when heated to temperatures greater than 300C. This characteristic is useful when vertical magnetic recording media is sputtered onto the nickel alloy deposit. Alloys used for vertical magnetics are sputtered at higher temperatures and for longer times than is commonly used for horizontal magnetic recording media. Sukonnick10 reports that "it was noticed that adhesion failures occur at Al/Ni interface, which points to the zincate as the weak link in the multi-layered structure." Good adhesion of EN deposits is essential for quality plating.
Good adhesion of EN deposits on aluminum results from excellent cleaning, minimum etching, proper zinc immersion chemistry, thin zinc films, the ability of the EN solution to dissolve the entire zinc layer, good filtration and proper EN bath chemistry. Proper maintenance of all the processing solutions is essential to good quality plating, including controlling hydrogen relief by properly selecting the rate of temperature ramp up, final temperature and length of time. For parts that are totally encapsulated, slower rates in temperature increase and longer times are needed. EN has no grain boundaries that allow quick escape of hydrogen. Control of intrinsic stress is necessary to lower the forces due to thermal movement.